Along with the spread of electric vehicles (EVs) and hybrid electric vehicles (HEVs), it is considered to install a power transmission pad for wireless charging to the EVs and HEVs in a charging station and like. The power transmission pad is covered with a protector such as cement and then embedded in the ground. Since a high voltage is applied to the power transmission pad, the protector is required to have excellent insulation performance. However, if the protector is formed with cement or the like, cracks occur due to aging degradation, and hence desired insulation performance may not be obtained. If the insulation performance is degraded, it is required to remove the protector and embed the power transmission pad again, which takes a high maintenance cost.